1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method and, more particularly, to a density adjustment process for generating an image composed of a copy-forgery-inhibited pattern image for warning against the use of copy products in an image processing system including an image processing apparatus, such as a computer, and a printer.
2. Description of the Related Art
Forgery-inhibited sheets having a special print pattern are used for forms and resident cards to inhibit or restrain copying. If a forgery-inhibited sheet is copied on a copying apparatus, a resulting sheet will bear the word “COPY”, for example, although the word on the original sheet is less visible to the human eyes. In this way, the original print sheet and the copy product thereof are distinctly discriminated. The use of the forgery-inhibited sheets causes any person to hesitate to use the copy product. Furthermore, the use of the forger-inhibited sheets psychologically warns persons against copying the original sheet. To distinctly discriminate between the original and the copy product, the character string on the original forgery-inhibited sheet is made less visible to the human eyes. If the character string “COPY” on the original sheet is clearly recognizable, the original can be identified as a copy product, and the forgery-inhibited sheet cannot function as an original.
Techniques for manufacturing such forgery-inhibited sheets are disclosed in U.S. Pat. No. 5,788,285 granted to Wicker and U.S. Pat. No. 6,000,728 granted to Mowry et al.
Such forgery-inhibited sheets, produced using a special printing technique, are costlier than standard commercial sheets of paper. A character that emerges on a copy product can be set on the forgery-inhibited sheet during manufacture only. The application of the forgery-inhibited sheet and the character string are thus subject to limitations. The convenience of manufacture is assured at the expense of flexibility of the known forgery-inhibited sheets in applications.
Contents, such as forms and resident cards, are also handled as digital data as more and more contents become digital. However, the introduction of digital techniques into the forms and resident cards is still at a transitional phase thereof. As a result, contents in the form of computer-generated digital data are still typically printed on sheets of paper on a printer.
Along with the rapid advance of printing performance, several techniques draw attention. In one technique, a sheet having the same effect as the known forgery-inhibited sheet is printed on-demand on a system of a computer and a printer. Japanese Patent Laid-Open No. 2001-197297 and Japanese Patent Laid-Open No. 2001-238075 disclose techniques in which copy-forgery-inhibited pattern images are superimposed on the background of computer-generated content data when the content data is printed using a printer. The copy-forgery-inhibited pattern image on an original (printed matter output from the printer) looks like a mere pattern or a mere background color to the human eyes. When the original is copied, a predetermined character emerges on a resulting copy product. This arrangement induces the same effect as the forgery-inhibited sheet on a forger.
When a computer-generated copy-forgery-inhibited pattern image is superimposed, standard print sheets of paper can be used. For this reason, this technique provides cost advantages over the forgery-inhibited sheet. During printing of content, a copy-forgery-inhibited pattern image can be produced. A character that is developed during copying is flexibly set. Dynamic information including the name of a user who has performed a print job and output date and time is designed to emerge as a character string.
As described above, a copy-forgery-inhibited pattern image such as a predetermined character, which is invisible prior to copying, emerges on a copy product, restrains the use of the copy product, and allows any persons to distinctly recognize that the sheet is a copy product not the original. To achieve this effect, the copy-forgery-inhibited pattern image includes two areas. In one area, an image remains (emerges) on the copy product, and in the other area, a density therewithin becomes less dense to the degree that an image therewithin is difficult to recognize. The two areas are substantially identical in density in the printed state thereof. In a macroscopic view, a word, such as “COPY”, becoming visible through copying, is hidden (embedded). In a microscopic view, the following different features are noticed at print dot levels.
Hereinafter, an image that emerges through copying is referred to as a “latent image”, and an image that disappears or becomes lighter is referred to as a “background”. The copy-forgery-inhibited pattern image typically contains a latent image portion and a background portion. The latent image is also referred to as a foreground, frequently used as a user interface term.
The copy-forgery-inhibited pattern image is not limited to the above-referenced structure. The copy-forgery-inhibited pattern image may be a character string, such as “COPY”, or a logo, or a pattern, each emerging (being developed) on the copy product. Even if the character string, such as “COPY”, is displayed in an outlined state, the character string achieves the copy forgery inhibition object. In this case, the character string “COPY” becomes a background image.
In dot printers, such as an electrophotographic printer and an ink-jet printer, an area where an image remains on a copy product (such as a latent image portion or a foreground image portion) is composed of a cluster of concentrated dots. An area (such as a background portion) that is reproduced at a density lower than a density of an area where an image disappears or remains through copying is composed of dispersed dots. The density of the entire copy-forgery-inhibited pattern image in the printed state thereof is substantially uniform by balancing both areas in density.
FIG. 18 illustrates the two areas. As shown, a copy-forgery-inhibited pattern image is constructed of a background area where dots are dispersed and a latent image portion where dots are concentrated. The two areas are produced using halftone dot process and dither process. When the copy-forgery-inhibited pattern image is generated using the half-tone process, a halftone process with a large number of lines is appropriate for the latent image portion and a halftone process with a small number of lines is appropriate for the background portion. When the copy-forgery-inhibited pattern image is generated using the dither process, a dither process using a dot clustered dither matrix is appropriate for the latent image portion while a dither process using a dot dispersed dither matrix is appropriate for the background portion.
Copying apparatuses are typically subject to a limit of reproduction capability that depends on an input definition and an output definition. Within the input definition, tiny dots of an original document are read, and within the output definition, tiny dots of an original document are reproduced. If the dots in the background portion of the copy-forgery-inhibited pattern image are formed in a size less than the dot size of the reproduction limit, and if the dots in the latent image portion are formed in a size larger than the dot size of the reproduction limit, an image composed of large dots in the copy-forgery-inhibited pattern image is developed but an image composed of small dots is not reproduced through copying. As a result, a latent image emerges. Even if the dispersed dots do not completely disappear through copying, and even if the density of the latent image is low in comparison with the concentrated dot cluster after copying, the latent image is relatively distinctly visible.
FIG. 19 illustrates the emergence of the latent image. The left-hand portion of FIG. 19 illustrates a copy-forgery-inhibited pattern image in the printed state thereof. The right-hand portion of FIG. 19 illustrates a copy product that is obtained by copying the image of the left-hand portion of FIG. 18. As shown, the latent image composed of a cluster of concentrated dots emerges, and the background image composed of dispersed dots disappears.
The copy-forgery-inhibited pattern image is not limited to the above arrangement. Any type of copy-forgery-inhibited pattern image is perfectly acceptable as long as a character string, such as “COPY INHIBITED”, emerges on a copy sheet so that users can clearly recognize the character string. For example, if a character string is specified as a background portion so that the character string emerges in an outlined form through copying, the purpose of the copy-forgery-inhibited pattern image is achieved.
When the copy-forgery-inhibited pattern image is preferably unrecognizable during a printing process. The density of one given unit area in the latent image portion and the density of another identical unit area in the background portion in the copy-forgery-inhibited pattern image are substantially equal to each other. The densities and the density relationships are subject to the print characteristic of a printer, temperature, humidity, and aging. When the copy-forgery-inhibited pattern image is printed, a mechanism for adjusting the density and the density relationship between the latent image portion and the background portion is used. For example, Japanese Patent Laid-Open No. 2001-197297 discloses a gradation calibration technique. According to the disclosure, a gradation calibration is performed so that a gradation of a latent image portion having undergone a halftone process and a gradation of a background portion having undergone no halftone process substantially match each other through printing. More specifically, the gradation calibration process is performed on one of a background portion having undergone the halftone process and a background portion having undergone no halftone process, or both.
In the copy-forgery-inhibited pattern image, the latent image portion is composed of a cluster of dots. On the other hand, the background portion is composed of dispersed small dots. It is difficult to perform density adjustment in a uniform fashion. Given the same amount of adjustment input in the density adjustment of the copy-forgery-inhibited pattern image, an actual response in density level is different from the latent image portion to the background portion.
Density adjustment is not easy and takes a lot of time in a known adjustment mechanism in which the density in the latent image portion is merely adjusted against the density in the background portion in the copy-forgery-inhibited pattern image.
The density adjustment of the copy-forgery-inhibited pattern image must be performed two-dimensionally on the latent image portion and the background portion taking into consideration a density relationship therebetween. It is necessary to find a desired density from within a wide range of densities. Since no common rule is applicable to a density adjustment input and a change responsive to the adjustment input in the latent image portion and the background portion, the finding of an appropriate density level is typically performed by trial and error.
The density ranges applicable to the latent image portion and the background portion in the copy-forgery-inhibited pattern image can be limited. If a density variation exceeds the limit, the density of the latent image portion and the density of the background portion cannot be set to be apparently equal to each other. The density adjustment thus becomes difficult.
When the copy-forgery-inhibited pattern image is output, an image composed of large dots (concentrated dots) constituting the latent image portion is reproduced at a relatively stable density while an image composed of small dots (dispersed dots) constituting the background portion is reproduced at a relatively unstable density. The unstable density is subject to physical characteristics as discussed below, and density changes in the latent image portion and the background portion in response to an amount of adjustment input become further different.
The physical characteristics are related to a print method. In the electrophotographic method, the size of each dot is different depending on a laser beam diameter and a voltage density. In the ink-jet print method, the size of each dot is different depending on an ejection nozzle diameter, and the volume of ink drops. In any case, the density of the background portion changes depending on these physical characteristics.
The density also greatly depends on operation environments under which the printers are used. For example, the size of dots changes depending on temperature and humidity. The size of dots also changes depending on the aging of the printer, and the degradation of a print mechanism.
A problem may arise when a density value is set using numerals on a user interface operation screen of an operation panel. For example, a user can recognize and memorize a simply set value as an “appropriate setting”. Regardless of a change in the actual density of the latent image portion and the background portion in the copy-forgery-inhibited pattern image, the user may set the memorized density value again. This leads to not only an inappropriate value setting but also to a secondary erroneous setting which is based on the wrong memorized density value. It may take a significant amount of time before the user becomes aware of the wrong setting.